Gallium arsenide substrate comprising a surface oxide layer with improved surface homogeneity

ABSTRACT

A gallium arsenide substrate which exhibits at least one surface having a surface oxide layer comprising gallium and arsenic oxides and which exhibits at least one surface having, according to an ellipsometric lateral substrate mapping with an optical surface analyzer, based on a substrate diameter of 150 mm as reference, a defect number of &lt;6000 and/or a total defect area of less than 2 cm 2 , wherein a defect is defined as a continuous area of greater than 1000 μm 2  having a deviation from the average measurement signal in elipsometric lateral substrate mapping with an optical surface analyzer of at least ±0.05%.

RELATED APPLICATIONS

This application is a continuation of a co-pending U.S. application Ser.No. 15/886,091 filed on Feb. 1, 2018, which is a Divisional of U.S. Ser.No. 14/767,603 filed on Aug. 13, 2015, now U.S. Pat. No. 10,460,924issued on Oct. 29, 2019, which is a US National Phase of PCT ApplicationNo. PCT/EP2014/052745 filed on Feb. 12, 2014, claiming the benefit under35 U.S.C. 119(a)-(d) from German application 10 2013 002 637.7 filed onFeb. 15, 2013. The contents and disclosures of these prior applicationsare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a process for producing asurface-treated gallium arsenide substrate as well as a gallium arsenidesubstrate as such and the use thereof.

BACKGROUND OF THE INVENTION

Gallium arsenide substrate wafers are used for the production ofhigh-frequency amplifiers and switches as well as light-emittingcomponents such as semiconductor lasers and luminescence diodes. Thecomponent structures (transistors and diodes) are typically producedfrom epitaxially deposited mixed crystal stacks of variable elementswhich can be selected from the group of Ga—In—As—P—Al—N, wherein in theindividual layers different electronic properties can be set at samecrystal lattice parameters by specific compositions. This kind oflattice-adapted epitaxy aims at a very high layer quality and low defectdensities. In this regard the layer quality depends not only on theconditions in the epitaxy process, but it is also influenced by thesubstrate properties.

The production of GaAs substrates starts with the growing of respectivebulk crystals and the subsequent singularization in a sawing/separatingprocess (see for example T. Flade et al., “State of the art 6″ SI GaAswafers made of conventionally grown LEC-crystals”, Journal of CrystalGrowth, 198-199(1), 1999, p. 336-342 and Th. Bünger et al., “Developmentof a vertical gradient freeze process for low EPD GaAs substrates”,Materials Science and Engineering B, 80(1), 2001, p. 5-9). Subsequentlythe substrates are treated in a multi-stage process, inter aliapolished, in order to obtain advantageous properties of geometry(thickness, curvature, wedge shape) and roughness (see for example Fladeet al.). After the last polishing step the pure highly reactive GaAssurface is exposed, and an oxide growth inevitably starts immediately.The typical subsequent cleaning in liquid media entails a sequence ofoxide forming and respectively oxide removing steps and primarily servesto decrease the number of particles and of residual impurities orrespectively residual contaminations on the substrate. During the lastcleaning steps and the subsequent drying of the substrate an oxide layeris formed. This oxide layer can still undergo change during the timeuntil the insertion of the substrates into the epitaxy apparatus (seefor example D. A. Allwood, S. Cox, N. J. Mason, R. Palmer, R. Young, P.J. Walker, Thin Solid Films, 412, 2002, p. 76-83).

The composition of the oxide layer can be measured for example by meansof X-ray-excited photoelectron spectroscopy (XPS) in which coreelectrons of the oxidized atom are spectroscopically investigated. Theoxidation states can be determined from the energetic shifts of theexcited electrons. The measurement methodology is described for theexample of GaAs in detail in C. C. Surdu-Bob, S. O Saied, J. L Sullivan,Applied Surface Science, Volume 183(1-2), 2001, p. 126-136. Depending onthe oxidation conditions the ratio of arsenic and gallium oxides liesbetween 1 and 5. Typically GaO, Ga₂O₃ and As₂O, AsO, As₂O₃, As₂O₅ occur(see for example F. Schröder-Oeynhausen, “OberflächenanalytischeCharakterisierung von metallischen Verunreinigungen and Oxiden aufGaAs”, Dissertation, University of Münster, 1996 and J. S. Song, Y. C.Choi, S. H. Seo, D. C. Oh, M. W. Cho, T. Yao, M. H. Oh, Journal ofCrystal Growth, 264, 2004, p. 98-103).

In order to influence the growth of epitaxial layers also by the surfaceproperties of the wafer surface, so far the possibility for the laterthermal desorption of surface layers (e.g. oxides) immediately beforethe start of the epitaxial processes in the epitaxy apparatus wasfrequently used. In this respect the roughness of a thermally desorbedsurface, the degree of contaminations and impurities (particles) play arole in the qualitative properties of the layer stacks to be depositedand the components produced therefrom. For III/V semiconductors thedependence of the quality of epitaxially deposited layers on parametersof the processes used for the preceding cleaning and surface propertiesassociated therewith were investigated. In this regard it was putativelypresumed that the wet chemical treatment on the whole surface of thewafer is homogenous in each of the steps.

The wet chemical cleaning of wafers is typically performed in successiveliquid baths. Process racks with wafers are usually set from bath tobath by means of automatic transport systems and finally dried. In mostcases the cleaning comprises a sequence of acidic and alkaline wet stepswith intermediate rinsing steps in deionized water (DI water). In thisrespect ammonium hydroxide (NH₄OH) or organic amines are usually used asalkaline components. Acids used are for example hydrogen fluoride (HF)and hydrogen chloride (HCl), further also sulfuric acid (H₂SO₄) ororganic acids. Often the cleaning media additionally contain additivessuch as for example oxidizing agents, surfactants (surface-activeagents) or chelating agents. For the removal of particles for exampleultrasound or megasound in individual baths is used. For the drying ofwafers rinsed with DI water in principle diverse processes areconceivable, in practice the drying conventionally relies on the removalof the DI water using the centrifugal forces during the rapid rotationof the wafers or respectively wafer carriers (spin drying) (see e.g.Song et al.).

However, the conventional processes do not provide gallium arsenidesubstrates which fulfill the increasing requirements on subsequentepitaxy processes in that the large-area and reliable epitaxialproduction of components with the needed layer quality and the requireddefect densities in appropriate yield is to be enabled.

The object of the present invention is to provide an improved processfor the production of gallium arsenide substrates exhibiting favorableproperties for a subsequent epitaxy.

SUMMARY OF THE INVENTION

For the characterization of the surface properties of differentlytreated gallium arsenide substrates the ellipsometric lateral substratemappings of an optical surface analyzer were used. Details of themeasurement method and the analysis methods used are given in thedescription of the preferred embodiments as well as in the descriptionof the Examples.

Without limiting the invention, in the following items are provideddescribing main aspects, preferred embodiments and particular featuresof the present invention:

1) A process for producing a surface-treated gallium arsenide substrate,the process comprising the steps:

a)providing a gallium arsenide substrate;b) oxidation treatment of at least one surface of the gallium arsenidesubstrate in dry condition by means of UV radiation and/or ozone gas;c) contacting the at least one surface of the gallium arsenide substratewith at least one liquid medium; andd) Marangoni drying of the gallium arsenide substrate.

2) The process according to item 1, wherein step c) comprises thefollowing steps:

i) contacting the at least one surface of the gallium arsenide substratewith alkaline aqueous solution, optionally applying megasound; andii) subsequently contacting the at least one surface of the galliumarsenide substrate with water.

3) The process according to item 1 or 2, wherein in step c) an alkalineaqueous solution is used which is a solution of NH₃ or organic amine inwater, preferably of NH₃, more preferably of NH₃ in a concentration of0.1-2 percent by volume and particularly preferably of NH₃ in aconcentration of 0.2-1 percent by volume.

4) The process according to item 2 or 3, wherein step c) furthercomprises the following steps:

iii) subsequent to step ii) contacting the at least one surface of thegallium arsenide substrate with acidic aqueous solution, optionally inthe presence of an oxidizing agent; andiv) subsequently further contacting the at least one surface of thegallium arsenide substrate with water, wherein preferably the water atleast initially contains a pH value modifying additive.

5) The process according to item 4, wherein the acidic aqueous solutionis a solution of HCl or HF in water, preferably in a concentration of0.1-0.5 percent by volume, more preferably in a concentration of0.1-0.25 percent by volume and particularly preferably is HCl in aconcentration of 0.15-0.25 percent by volume.

6) The process according to item 4 or 5, wherein the oxidizing agent inthe acidic aqueous solution is ozone or H₂O₂, preferably ozone, morepreferably ozone in a concentration of 10-50 ppm and particularlypreferably ozone in a concentration of 30-50 ppm.

7) The process according to one of the items 4-6, wherein the pH valuemodifying additive is basic or acidic, preferably basic, more preferablyNH₃, even more preferably NH₃ in a concentration of 0.01-0.2 percent byvolume and particularly preferably NH₃ in a concentration of 0.05-0.1percent by volume.

8) The process according to one of the items 4-7, wherein in step c)subsequent to step iv) further steps according to the steps i) and ii)are carried out.

9) The process according to one of the items 2-8, wherein water isdeionized water or ultra-pure water.

10) The process according to one of the preceding items, wherein thegallium arsenide substrate provided in step a) was beforehandsingularized or respectively separated from a gallium arsenide bulkcrystal and/or was polished and preferably precleaned, more preferablywet chemically pre-cleaned and particularly preferably precleaned wetchemically and with brush scrubbing.

11) The process according to one of the preceding items, wherein thegallium arsenide substrate provided in step a) is doped or undoped.

12) The process according to one of the preceding items, wherein in stepd) an aqueous isopropanol solution is used.

13) A process for producing a plurality of surface-treated galliumarsenide substrates, wherein in the process according to one of thepreceding items a plurality of gallium arsenide substrates issimultaneously subjected to the respective steps b)-d).

14) A gallium arsenide substrate which exhibits at least one surfacehaving in ellipsometric lateral substrate mapping with an opticalsurface analyzer a variation of the laterally resolvedbackground-corrected measurement signal whose distribution percentile of1% normalized to the substrate average of the phase shift signal isgreater than −0.0065.

15) The gallium arsenide substrate according to item 14, which exhibitsat least one surface having in ellipsometric lateral substrate mappingwith an optical surface analyzer a variation of the laterally resolvedbackground-corrected measurement signal whose distribution percentile of1% normalized to the substrate average of the phase shift signal isgreater than −0.0060, preferably greater than −0.0055, more preferablygreater than −0.0050, even more preferably greater than −0.0045,particularly preferably greater than −0.0040, particularly greater than−0.0030, in particular greater than −0.0020, even greater than −0.0010and up to 0.0000 excluding 0.0000.

16) A gallium arsenide substrate which exhibits at least one surfacehaving in ellipsometric lateral substrate mapping with an opticalsurface analyzer, based on a substrate diameter of 150 mm as reference,a defect number of <6000 and/or a total defect area of less than 2 cm²,wherein a defect is defined as a continuous area greater than 1000 μm²having a deviation from the average measurement signal in ellipsometriclateral substrate mapping with an optical surface analyzer of at least±0.05%.

17) The gallium arsenide substrate according to item 16, wherein thedefect number is <5000, preferably <4000, more preferably <3000, evenmore preferably <2000, yet more preferably <1000, still more preferably<500, still more preferably <300, still more preferably <250, still morepreferably <200, still more preferably <150 and particularly preferably<100, and/or the total defect area is less than 1 cm², preferably lessthan 0.5 cm², even more preferably less than 0.1 cm², even morepreferably less than 0.05 cm², still more preferably less than 0.01 cm²,still more preferably less than 0.005 cm² and particularly preferablyless than 0.0035 cm².

18) A gallium arsenide substrate which is produced according to theprocess according to one of the items 1-12.

19) The gallium arsenide substrate according to one of the items 14-18,wherein the diameter is at least 100 mm, preferably at least 150 mm andmore preferably at least 200 mm.

20) A polished and surface-finished gallium arsenide substrate having adiameter of at least 150 mm, wherein the surface treatment comprises anoxidation treatment of at least one surface of the gallium arsenidesubstrate in dry condition by means of UV radiation and/or ozone gas, acontacting of the at least one surface of the gallium arsenide substratewith at least one liquid medium and a Marangoni drying of the galliumarsenide substrate.

21) The gallium arsenide substrate according to item 20, having athickness of not greater than approximately 600 μm or respectively notless than approximately 800 μm.

22) The gallium arsenide substrate according to item 20 or 21, whereinthe thickness lies in a range from approximately 100 to approximately600 μm or respectively the thickness is greater than approximately 800μm and the thickness preferably lies in a range from approximately 250to approximately 500 μm or respectively from approximately 800 toapproximately 2000 μm.

23) A polished and surface-finished gallium arsenide substrate,exhibiting a thickness of not greater than approximately 600 μm.

24) A polished and surface-finished gallium arsenide substrate having athickness of not less than approximately 800 μm.

25) The polished and surface-finished gallium arsenide substrateaccording to one of the items 20 to 24, wherein the treated surface ofthe substrate exhibits the properties defined in one of the items 14 to17.

26) The gallium arsenide substrate according to one of the items 14-25,wherein said gallium arsenide substrate is doped or undoped.

27) A gallium arsenide substrate which exhibits at least one surfacehaving within 9 months, preferably 12 months, after the production asubstantially not deteriorating, preferably a not deteriorating,variation of the laterally resolved background-corrected measurementsignal in ellipsometric lateral substrate mapping with an opticalsurface analyzer.

28) The gallium arsenide substrate according to one of the items 14-26,wherein the at least one surface within six months after the productionexhibits a substantially not deteriorating, preferably a notdeteriorating, variation of the laterally resolved background-correctedmeasurement signal in ellipsometric lateral substrate mapping with anoptical surface analyzer.

29) A plurality of gallium arsenide substrates, which are producedaccording to the process according to one of the items 1-13 and exhibitamong one another a substantially same, preferably same, variation fromsubstrate to substrate of the laterally resolved background-correctedmeasurement signal in ellipsometric lateral substrate mapping of therespective at least one surface with an optical surface analyzer.

30) The gallium arsenide substrate according to one of the items 14 to17 and 25 to 28 or respectively the plurality of gallium arsenidesubstrates according to item 29, wherein ellipsometric lateral substratemapping with an optical surface analyzer is carried out with an opticalsurface analyzer analogous to Candela CS20, preferably specifically withan optical surface analyzer Candela CS20, more preferably with anoptical surface analyzer using laser light having a wavelength of 405 nmand whose optical path comprises a half-wave plate, a quarter-waveplate, a polarization-sensitive beam splitter and two detectors, evenmore preferably with an optical surface analyzer according to the phaseshift channel of Candela CS20 and in particular with an optical surfaceanalyzer according to FIG. 1.

31) Use of the gallium arsenide substrate according to one of the items14-28 and 30 or respectively of the plurality of gallium arsenidesubstrates according to item 29 for epitaxial crystal growth, optionallyafter storage and preferably without pretreatment after providing thegallium arsenide substrate and before the epitaxial crystal growth.

32) Use of the gallium arsenide substrate according to one of the items14-28 and 30 or respectively of the plurality of gallium arsenidesubstrates according to item 29 for the production of semiconductorcomponents or electronic and optoelectronic components.

33) Use of the gallium arsenide substrate according to one of the items14-28 and 30 or respectively of the plurality of gallium arsenidesubstrates according to item 29 for the production of power components,high-frequency components, light-emitting diodes and lasers.

34) Use of an optical surface analyzer, particularly an optical surfaceanalyzer analogous to Candela CS20 or specifically of the Candela CS20,for the optical contact-free quantitative characterization of thehomogeneity of surface properties, particularly for the quantitativecharacterization of the homogeneity of the surface oxide layer, ofgallium arsenide substrates by means of ellipsometric lateral substratemapping.

35) The use according to item 34, wherein a laterally resolvedmeasurement signal is corrected in regard to a background having lowerfrequency by means of discrete complex Fourier transformation,preferably using the Levenberg-Marquardt algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the optical setup and optical path of anoptical surface analyzer according to Candela CS20 used for theellipsometric surface measurements, in particular for the so-calledphase shift channel of the measurement instrument Candela CS20.

FIG. 2 shows typical ellipsometric lateral substrate maps, so-calledCandela images, for different final cleaning technologies (from left toright: Comparative Example 1, Example 1 and Example 2).

FIG. 3 exemplarily illustrates the measured signal curve (“raw data”) ofa circular track, wherein the curve “background” describes thecrystallographically caused part of the measurement signal, wherein thispart can be modeled by Fourier transformation using theLevenberg-Marquardt algorithm.

FIG. 4 exemplarily shows determined phase shifts of a track aftersubtraction of the crystallographically caused background signal.

FIG. 5 shows typical ellipsometric lateral substrate maps, so-calledCandela images, after subtraction of the crystallographically causedbackground signal for different final cleaning technologies (from leftto right: Comparative Example 1, Example 1 and Example 2).

FIG. 6 presents typical global frequency distributions of thebackground-corrected phase shifts, i.e. residuals of thebackground-corrected mappings, for respectively individual waferssubjected to respectively different final cleaning technologies.

FIG. 7 shows typical 1% percentiles of in each case 25 wafers producedaccording to the methods from the Comparative Examples 1 and 2 orrespectively the Examples 1 and 2 according to the invention. For anobjective comparison of the percentiles said 1% percentiles arenormalized to the respective wafer averages of the phase shift signals.

FIG. 8 shows typical so-called defect maps of background-correctedCandela images for different final cleaning technologies (from left toright: Comparative Example 1, Example 1 and Example 2).

FIG. 9 shows typical defect numbers and defect areas of in each case 25wafers produced according to the methods from the Comparative Examples 1and 2 or respectively the Examples 1 and 2 according to the invention.

FIGS. 10-12 show flow diagrams of different wet chemical cleaningtreatments of GaAs wafers, in accordance with embodiments of thedisclosure.

DETAILED DESCRIPTION OF THE INVENTION

Without limiting the present invention thereby, in the following theinvention is illustrated by the detailed description of the Figures,aspects, embodiments and particular features, and particular embodimentsare described in more detail.

A first aspect of the present invention provides a process for producinga surface-treated gallium arsenide substrate which comprises thefollowing steps:

Providing a gallium arsenide substrate, oxidation treatment of at leastone surface of the gallium arsenide substrate in dry condition by meansof UV radiation and/or ozone gas, contacting of the at least one surfaceof the gallium arsenide substrate with at least one liquid medium andMarangoni drying of the gallium arsenide substrate.

In the process according to the invention it was surprisingly found thatby the combination of the coordinately aligned dry oxidation treatment,the contacting with liquid medium and Marangoni drying the surfaceproperties of the gallium arsenide substrate can be favorably and veryhomogeneously set, in particular the surface oxide. As a consequence,contrary to conventionally produced substrates, the surface oxide on theentire substrate surface can be thermally desorbed in a controlled andreproducible manner and practically from the entire substrate surface,for example immediately before the epitaxy in the epitaxy apparatus. Inthis way the substrate according to the invention can be used in theepitaxy process without further treatment. Here the gallium arsenidesubstrate can be a wafer.

In the present invention it was advantageously recognized that a veryhomogeneous surface is required since the thermal desorption behaviourand the thermal desorption temperatures of different oxides varysubstantially and the thermal desorption behaviour of the oxide layer ona GaAs substrate depends on its composition of gallium and arsenicoxides of different oxidation states as well as the oxide layerthickness. Another advantage resides in that with the complete thermaldesorption of the homogeneous oxide layer roughenings of the surfaceduring the desorption of this oxide layer and thereby morphologicalperturbations in the epitaxially grown layer structures can beprevented. In the present invention it was further recognized that for aconsistently high quality of the epitaxy layers over the entire wafersurface and hence for high yields the lateral surface homogeneity orrespectively oxide homogeneity, that is the lateral uniformities ofoxide composition and oxide thickness, are of great importance. Inparticular it was recognized that on parts of the GaAs surface having,compared to the rest of the wafer surface, a strongly differing,unfavorable oxide composition or oxide thickness a disturbed thermaldesorption behaviour of the oxide can occur, whereby in turn remainingoxide islands or a stronger roughening of the wafer surface can result.The risk for the formation of crystallographic perturbations in theepitaxy layers can be very large at the disturbed regions, whereby theelectrical functionality of the components produced from the GaAs wafercan be affected. It was furthermore advantageously recognized that forthe setting of the surface properties the execution of the last stepswithin the production process of GaAs wafers, namely the final polishingand the subsequent cleaning steps, and thereof in particular the lastcleaning steps immediately before the drying and the drying of thewafers, is of particular importance.

In consideration of the subsequent wet treatment and the specific dryingstep carried out thereafter it is of particular importance that for theproduction and cleaning process according to the present invention thesurface of the GaAs substrate is homogeneously oxidized in dry conditionbefore the contacting with liquid medium. The dry oxidation treatmentaccording to the invention is carried out in one case by irradiation ofthe surface with UV light, preferably UV light of short wavelength. Onthe one hand the oxidation is initiated by the energy of the UV lightitself. Furthermore, oxygen from the surroundings is partly converted bythe light energy into ozone which enhances the oxidation process on thewafer surface. In another case the dry oxidation occurs through theexposure to ozone gas. The applied ozone gas can be produced by an ozonegenerator. Herein the dry oxidation effects the formation of ahomogeneous passivating oxide layer as well as the oxidative destructionof organic impurities on the surface of the substrate. In the process ahomogeneous surface oxide layer which in the subsequent steps serves as“sacrificial oxide” is formed in a controlled manner, laterally as wellas depth-wise. A deep oxidation of undefined surface states produced bythe preceding process steps as well as the prevention of selectivelyacting oxide formation mechanisms which would occur in natural or wetchemical oxidation for the formation of a “sacrificial oxide” can beparticularly advantageous. In the dry oxidation treatment the GaAssurface is oxidized more strongly towards bulk GaAs since the oxide onthe front side grows partly in the direction of the wafer back side.Regions having surface properties differing from the rest of the wafersurface stemming from the preceding processes, which in the followingare denoted as inhomogeneity defects, can subsequently be removed fromthe wafer surface more easily in the oxidized state, for example by wetchemical processes. A high quality of the oxide layer produced in thefirst step of the cleaning process is a prerequisite for the subsequentapplication of the contacting liquid medium, preferably a mildly erosiveand roughness-neutral etching step. In the present invention it wasfurthermore particularly recognized that the dry oxidation isexcellently suited to homogeneously hydrophylize the entire surface.This is particularly advantageous in view of a uniform wetting of thewafer surface during the transfer of the wafers between cleaning baths.

It was furthermore found that it is difficult to completely removeoxides from the GaAs surface which were created wet chemically. Thehomogeneity of the produced oxide layer and hence also (the degree andmanner of) the removability of the produced oxide layer aresignificantly influenced by the oxidation conditions.

Subsequent to the dry oxidation treatment and the contacting of thesurface with liquid medium, for example for suitable cleaning steps,according to the present invention the surface wetted with liquid mediumis thereafter advantageously dried by Marangoni drying. Marangoni dryingis based on the Marangoni principle. In the process a suitable agent isused, for example alcohol or other organic compounds, in particularisopropanol, which after its concentration at the surface of the liquidmedium, for example at the water surface, reduces the surface tension ofthe liquid. When this liquid front, preferably the water front orrespectively aqueous front, is moved relative to the surface of thewafer standing for example upright in the bath the gradient in thesurface tension between the thin meniscus layer in contact with thewafer and regions of the liquid surface, preferably the water surface orrespectively aqueous surface, further apart from the wafer causes aresidue-free flowing-off of the liquid from the wafer surface.

Furthermore, the drying applied according to the invention isparticularly advantageous for the cleaning of GaAs wafers withparticularly small or particularly large thickness. In the Marangonidrying the wafers are subjected to less mechanical stress compared tospin drying which is generally used for the drying. In the presentinvention it was found that the risk and the tendency of wafer breakagefor thin wafers during the cleaning of GaAs wafers according to theprocess of the present invention are considerably reduced. For thickGaAs wafers during the spin drying unbalance can occur at highrevolution speeds due to the flats or notches applied for the marking ofthe crystallographic orientation, which likewise increases the risk ofwafer breakage. Therefore, the present cleaning process also offersadvantages for the cleaning of particularly thick GaAs wafers. Inparticular for the cleaning of large-area GaAs wafers, independent ofthe wafer thickness, still another advantage of the present inventionfollows: In the spin drying water residues are transported over thewafer surface up to the edge of the GaAs wafer which can lead to locallyconfined traces in the surface composition, so-called watermarks. In theMarangoni drying the drying always occurs immediately at thethree-phase-boundary line between wafer, liquid level, preferably waterlevel or respectively aqueous level, and the surrounding gas atmosphere.As a consequence, watermarks can be prevented for all substratethicknesses and the GaAs substrate wafers can be dried homogeneously.

It has furthermore been surprisingly seen that in combination with thepreceding dry oxidation treatment in the Marangoni drying in the presentprocess an extremely homogeneous oxide layer is produced on the treatedGaAs substrate surface with respect to thickness and composition. Thisis very advantageous for conventionally usual GaAs substrate thicknessesbut in particular also for relatively thin or even extremely thin orrespectively for relatively thick or even extremely thick GaAssubstrates with simultaneously large diameter or respectively for largediameters and all substrate thicknesses. In this respect large diametersare at least 100 mm, preferably at least 150 mm and more preferably atleast 200 mm. Substrate thicknesses can suitably be ≥100 μm; the maximumthickness can be determined by the desired application of a substrate,for example up to 5000 μm, usually preferably reaching up to 2000 μm orup to 1000 μm. In this way the cleaning sequence according to theinvention enables the production of large-area GaAs wafers of variousthickness having homogeneous surfaces.

The provided gallium arsenide substrate can be doped or undoped, iscrystalline and particularly preferably mono-crystalline, wherein thesubstrate can be produced by singularization or respectively separationfrom a GaAs bulk single-crystal body (ingot, boule). In an embodimentsaid provided gallium arsenide substrate was polished beforehand,preferably polished and subsequently pre-cleaned, more preferablypolished and subsequently wet chemically pre-cleaned and particularlypreferably polished and subsequently pre-cleaned wet chemically and withbrush scrubbing. In this respect brush scrubbing can for example beadvantageous to remove particles without the addition of organicsurfactants. In the process according to the invention contaminations byparticles and surface defects such as scratches, pits or bumps, crystaldefects and severe roughnesses are prevented or greatly reduced. Owingto the achieved high homogeneity of the cover layer (the oxide layer),the produced GaAs wafer is particularly suited as substrate for theepitaxial growth of layers and layer systems, and, apart from the usualthermal desorption, can be immediately—optionally after intermediatestorage—used for the epitaxy.

In a particular embodiment of the process according to the invention instep c) the following steps are comprised: The at least one surface ofthe gallium arsenide substrate is contacted with alkaline aqueoussolution, optionally applying megasound, and subsequently with water. Inparticular possibly still present inhomogeneities in the oxide layerwhich lie very close to the surface of the wafer can advantageously andsafely be removed.

In the process the wet cleaning with an alkaline cleaning step,optionally applying megasound, and with a water rinsing, preferably withdeionized water (DI water) or ultra-pure water is adjusted to thefurther process steps. During the transfer of the wafers betweendifferent baths of an automatically or manually operated wet processsystem a hydrophilic wafer surface is completely wetted with a liquidfilm.

In principle the prevention of dry regions on the wafer surface duringthe transport operations between the wet baths of a wet bench can alsobe achieved by carrying out the processes including the dryingpreferably in a single bath.

For this preferred embodiment of the process according to the inventiona controlled removal of the oxide layer occurs by the alkaline cleaningappropriately in a first liquid bath. For the setting of the pH value inthe simplest case ammonia, but also organic amine compounds can be addedto the DI water. The concentration of the added chemicals can lie in therange of greater than 0.1 mass percentage (Ma %). The input of megasoundin this bath supports the removal of particles adhering to the surface.In this regard megasound can be delivered to the bath either by applyingappropriate transducers on a quartz glass tank from the outside or byapplying appropriately coated vibrating elements directly in the bath.

In principle a removal of the oxide layer with simultaneous particleremoval can alternatively also occur in an acidic cleaning medium.

When the oxide layer is removed in the alkaline medium, the high surfaceenergy of the wafer surface is maintained by the increased presence ofOH groups. The accordingly hydrophilic wetting behaviour of the surfacestabilizes the surface homogeneity not only in the following transfersteps but also leads to a higher uniformity of the GaAs surface in theMarangoni drying.

According to a preferred embodiment subsequent to the alkaline cleaningpreferably a rinsing of the wafers with DI water occurs. The processtime of the rinsing can lie in the range from few seconds to severalminutes depending on the respectively used concentration of thechemicals. The subsequent drying of the wafers according to theMarangoni principle is capable of effecting a very uniform drying of thewafer surface. Basic principle is the depression of the surface tensionof the water or respectively the aqueous solution by the appropriateinput of an appropriate agent such as for example isopropanol. The speedat which the wafers or respectively the water level or respectivelyaqueous level are moved can suitably be adjusted with respect to thesurface energy of the wafers to be dried. In the process an optimumbetween quality and wafer throughput can be obtained. For GaAs substratein the final stage of the wafer production appropriate drying speeds liein the range of a few hundredths millimeters per second up tocentimeters per second.

GaAs wafers dried according to the Marangoni process exhibit fewertraces of surface inhomogeneities compared to spin-dried wafers. Thisapplies to both hydrophobic and hydrophilic spin-dried wafers.

The roughness of the GaAs surface is not changed by the removal of theoxide in the alkaline cleaning step and it remains at approximately 0.3nm for R_(a). Also the metallic surface contaminations determined bymeans of total reflection X-ray fluorescence (TXRF) analysis remain onthe same level.

The cleaning sequence leads to an average particle contamination of atmost 10 particles with a diameter in excess of 0.3 μm per substrate(KLA-Tencor Surfscan 6420). In this way with a cleaning sequenceaccording to the preferred embodiment of the present invention thesurface homogeneity is further improved. Roughness and metalcontamination remain on the same level and the particle contamination islow.

GaAs wafers with a highly sensitive surface can be processed in thepatch process with wet chemical cleaning using different types of wafercarriers. Since in the present invention preferably large-area GaAswafers with a high surface homogeneity are produced, the use of wafercarriers which screen the wafer surface as little as possible ispreferable. An even better possibility is to work completely withoutcarrier. For the same reasons of an as homogeneous as possible wettreatment the flow conditions in the liquid baths are favorably set.

A preferred embodiment is exemplarily shown in the flowchart shown inFIG. 10.

Another embodiment of the process according to the invention is providedsuch that step c) further comprises the following steps: Subsequent tostep ii) contacting of the at least one surface of the gallium arsenidesubstrate with acidic aqueous solution, optionally in the presence of anoxidizing agent, and subsequent further contacting of the at least onesurface of the gallium arsenide substrate with water, wherein preferablythe water at least initially contains a pH value modifying additive.

The etching medium here consists of an acid and optionally an oxidizingagent. Also this embodiment leads after the final drying of the waferwith the Marangoni process to a very uniform surface oxide.

Here hydrochloric acid or hydrofluoric acid can be used. H₂O₂ or ozone(O₃) dissolved in the liquid medium serve as oxidizing agent. In thisfurther preferred embodiment all steps are coordinatively aligned suchthat i) inhomgeneities in the oxide layer and additionally also in theuppermost GaAs atom layers of the surface are removed, ii) by continuouscomplete wetting of the wafer surface during the transfer operationsbetween the wet process steps no new inhomogeneity defects are produced,iii) by the specific configuration of the last rinsing step all oxidetraces of the acidic cleaning step are uniformly removed and the GaAssurface is hydrophilized for the drying, and iv) by the Marangoni dryingan extremely uniform wafer surface is obtained.

The acidic cleaning serves inter alia to remove metallic contaminations.By the selection of the concentrations of acid and oxidizing agent,considering the high reactivity of the GaAs surface, the strength of thereaction and thus the etching abrasion as well as the roughening of thesurface can be controlled. On the basis of the high reactivity of theGaAs surface the conditions are advantageously selected such thatsubstantially no, preferably no, locally different oxide thicknesses,oxide compositions or roughenings, also inhomogeneity defects, form onthe surface of the GaAs wafers.

In the present invention suitable conditions and compositions for theoxidizing acidic cleaning step can be selected for which such apreferable abrasion of possible inhomogeneity defects from the precedingprocesses is possible on GaAs wafers, wherein in this cleaning step theroughness after the last polishing step is at least preserved.

Typical oxidizing agents for the cleaning of semiconductor wafers inaqueous solutions are hydrogen peroxide and ozone. As oxide dissolvingacids preferably hydrofluoric acid or hydrochloric acid are used.

According to this preferred embodiment of the present invention incomprehensive investigations conditions were found for which a removalof possible inhomogeneity defects is possible, the re-contamination byparticles remains low and the roughness obtained in the last polishingstep remains constant. For this purpose the acid concentrationpreferably is less than 0.25% and greater than 0.1%, determined aspercent by volume in the whole liquid. The rate of the material abrasioncan be determined substantially by the concentration of the oxidizingagent. This way a typical R_(a) roughness of 0.30±0.03 nm after the lastpolishing step can be improved up to 0.20±0.03 nm in the entire ozoneconcentration range.

The etching abrasion to be preferably carried out according to thisembodiment occurs on the basis of simultaneous oxide formation and oxideremoving processes. When the wafers are extracted from the acidicetching liquid for the transfer to the following liquid bath, they arecovered on their surface with an oxide layer. Due to the simultaneouslyhydrophobizing effect of the acid in this case the wafers are notautomatically completely wetted during the transfer despite the oxidelayer. Rather also the wetting behaviour after the acidic cleaning stephere is advantageously determined by the balance between acid andoxidizing agent. A good wetting behaviour of the GaAs wafers is requiredalso during this transfer operation in order to prevent for the reasonsmentioned above local perturbations of the oxide homogeneity in the dryregions. In the investigations carried out it was seen that also for thereason of good wettability the acid concentration is preferably keptbelow 0.25%.

Using more than 20 ppm ozone and HCl in low concentrations below 0.5% aswell as applying appropriate transfer times, during the transfer theGaAs wafers in air remain completely wetted. From this follows theparticularly preferable configuration for the acidic cleaning step inthis embodiment of the present invention. Particularly favorable is theuse of HCl with a concentration between 0.15 and 0.25% together withozone in a concentration of 10-100 ppm and a dwell time of the wafers inthis solution between 2 minutes and 5 minutes.

More preferably, after the acidic cleaning in the presence of anoxidizing agent, the subsequent cleaning step is carried out withaddition of a pH value modifying substance (“spiking”). Such anadditional step can be suitably adjusted with respect to the precedingacidic cleaning step as well as the subsequent Marangoni drying, and itcan contribute to prevent a possible regeneration of inhomogeneities. Byadding an acidically or basically acting substance to the circulationloop of the rinsing bath, the oxide layer on the GaAs wafer isimmediately removed uniformly during the immersion of the wafer.Possible inhomogeneity defects are prevented using ammonia but also whenapplying HCl. After the pH modified rinsing water circulates around thewafers for a short time, preferably fresh water is delivered and theactual rinsing of the wafers with fresh DI water starts. For the removalof the oxide layer in the rinsing step acidically as well as basicallyacting chemicals are suitable. The roughness of the GaAs surface is notincreased during the gently abrasive etching in the preferredconcentration region of the pH modifying substance with respect to theroughness before the cleaning sequence and it remains at least attypically 0.3±0.03 nm for R_(a).

According to the invention it can be achieved that the metallic surfacecontaminations determined by means of TXRF remain on the same low level.The average particle contamination with this cleaning sequence is around10 particles having a diameter in excess of 0.3 μm per wafer (KLA-TencorSurfscan 6420). This way with a cleaning sequence according to thispreferred embodiment of the present invention the surface homogeneity isimproved. Roughness and metal contamination remain at least on the samelevel and the particle contamination is low.

This particularly preferable embodiment is exemplarily shown in aflowchart shown in FIG. 11.

In a further embodiment of the process according to the invention instep c) subsequent to step iv) further steps according to steps i) andii) are carried out. In this embodiment after acidic cleaning andsubsequent rinsing the wafers are subjected before the drying yet afurther alkaline cleaning, optionally and preferably with exposure tomegasound. This way a still more effective particle removal is possiblecompared to the application of megasound in the DI water rinsing bath.Furthermore, the hydrophilic character of the GaAs surface is increasedby the alkaline treatment which has a favorable effect on the behaviourof the wafers during drying.

This embodiment is exemplarily shown in a flowchart shown in FIG. 12.

Another aspect of the present invention is a process for producing aplurality of surface-treated gallium arsenide substrates, whereinsimultaneously a plurality of gallium arsenide substrates in the processaccording to one of the preceding items is subjected to the respectivesteps b)-d).

The batch-wise production of a plurality of wafers enables on the onehand an economic production, but on the other hand also a highhomogeneity of the product properties, in particular the surfaceproperties, of the substrates among one another.

A further aspect of the present invention is a gallium arsenidesubstrate which exhibits at least one surface having in ellipsometriclateral substrate mapping with an optical surface analyzer a variationof the laterally resolved background-corrected measurement signal whosedistribution percentile of 1% normalized to the substrate average of thephase shift signal, i.e. for the distribution the 1% percentile, isgreater than −0.0065, preferably greater than −0.0060, more preferablygreater than −0.0055, even more preferably greater than −0.0050, stillmore preferably greater than −0.0045, particularly preferably greaterthan −0.0040, in particular greater than −0.0030, especially greaterthan −0.0020, preferably greater than −0.0010 and even up to 0.0000,wherein the absolute value 0.0000 can be excluded.

Phase shift signal denotes a measured or respectively detected signalwhich is based on phase-dependent properties of differently polarizedlight components, for example of perpendicularly and parallel polarizedlight. Preferably the phase shift signal is measured as intensitydifference between detector signals, more preferably two detectorsdetect differently polarized light components of laser light reflectedfrom the substrate surface, wherein the differently polarized lightcomponents after interaction of laser light and substrate areadditionally spatially separated.

The substrate average of the phase shift signal is the arithmetic meanof all phase shift signals measured on a substrate.

All deviations from the ideal homogeneous surface (residuals of thebackground-corrected mapping of the phase shift signals) can bepresented in a histogram. Since this distribution typically does notcorrespond to a Gaussian normal distribution, the frequently usedstandard deviation cannot be used as a measure of homogeneity. Rather itis expedient to define and to compute appropriate percentiles. For theevaluation of the homogeneity of the GaAs surfaces according to thepresent invention, the percentiles P1 or respectively Q_(0.01) are usedfor comparisons. Distribution percentile of 1% or respectively 1%percentile for the distribution denotes the percentile rank P1 belowwhich lie 1% of the total number of values of the distribution, and itcorrespond to the quantile Q_(0.01) (see for example F. Schoonjans, D.De Bacquer, P. Schmid P, “Estimation of population percentiles”.Epidemiology, 22, 2011, p. 750-751). In order to compensate forfluctuations of the irradiated light intensity the determinedpercentiles are normalized to the respective wafer averages of the phaseshift signal, i.e. they are divided by the arithmetic mean of all phaseshift signals measured on a substrate.

Another aspect of the present invention is a gallium arsenide substratewhich exhibits at least one surface having in background-correctedellipsometric lateral substrate mapping with an optical surfaceanalyzer, with respect to a substrate diameter of 150 mm as reference, adefect number of <6000, preferably <5000, more preferably <4000, evenmore preferably <3000, still more preferably <2000, yet more preferably<1000, still more preferably <500, still more preferably <300, stillmore preferably <250, still more preferably <200, still more preferably<150 and particularly preferably <100 and/or a total defect area of lessthan 2 cm², preferably less than 1 cm², more preferably less than 0.5cm², even more preferably less than 0.1 cm², still more preferably lessthan 0.05 cm², still more preferably less than 0.01 cm², still morepreferably less than 0.005 cm² and particularly preferably less than0.0035 cm², wherein a defect is defined as a continuous area greaterthan 1000 μm² having a deviation from the average measurement signal inellipsometric lateral substrate mapping with an optical surface analyzerof at least ±0.05%. With the designated measurement values an adequatedifferentiation of the gallium arsenide substrates according to theinvention, determinable analytically by means of the describedellipsometric lateral substrate mapping, versus conventional galliumarsenide substrates is given (see e.g. also the Examples describedbelow).

The ellipsometric lateral substrate mapping is preferably carried outwith an optical surface analyzer analogous to Candela CS20, morepreferably specifically with the optical surface analyzer Candela CS20.In particular optical surface analyzers are suitable for which the usedanalysis laser light uses a wavelength of 405 nm and whose optical pathcomprises a half-wave plate, a quarter-wave plate, apolarization-sensitive beam splitter and two detectors. Specifically anoptical surface analyzer can for example operate according to the phaseshift channel of Candela CS20; a typically usable configuration for apreferable optical surface analyzer is shown in FIG. 1.

Ellipsometry is based on the interaction of polarized light during itspropagation in optically active media. The selection of an appropriatelight wavelength as well as the measurement configuration in reflectionenable a high surface sensitivity. FIG. 1 schematically shows theoptical setup and optical path of an optical surface analyzer accordingto Candela CS20 used for the ellipsometric surface measurements, i.e.for ellipsometric lateral substrate mappings, of the present invention,in particular for the so-called “phase shift” channel of the measurementapparatus Candela CS20 from the company KLA-Tencor. Here for thecharacterization of GaAs substrate surfaces the polarized light of alaser having a wavelength of 405 nm is used which after passing througha half-wave plate is directed by a mirror and a focussing lens onto thesubstrate surface at an angle (θ) of 60° with respect to the normal. Theperpendicular and parallel polarized components are reflected at thesubstrate surface according to the optical properties of the oxide layerand are sent through a collecting lens, via a mirror and a quarterwave-plate onto a polarization-sensitive beam splitter. Here thedifferently polarized light components resulting from the interactionwith the substrate surface are separated and analyzed in the detectorsD1 and D2. According to a preferred embodiment the intensity differencebetween the detector signals is denoted as phase shift signal, and thephase shift signal (“phase shift”) characterizes the optical propertiesof the reflecting substrate surface. By means of rotation of the waferand simultaneous radial movement of the optical measurement system thecomplete surface of the wafer can be scanned in a spiral pattern. Byfast ellipsometric mapping or respectively scanning complete highlyresolved mappings or respectively images of the optical properties canalso be generated for large-area GaAs substrates (substrate mappings)for the assessment of the surface homogeneity. According to theinvention an optical surface analyzer analogous to Candela CS20 from thecompany KLA-Tencor can be used, i.e. a measurement device correspondingto a Candela CS20, in particular however the apparatus Candela CS20 ispreferably used (see also L. Bechtler, V. Velidandla, Proc. SPIE 4944,Integrated Optical Devices: Fabrication and Testing, 109, 2003;doi:10.1117/12.468295 and F. Burkeen, Compound Semiconductor, 14 (10),2008), in particular the phase shift channel of Candela CS20. Inprinciple however similar or different ellipsometric measurement devicesand mapping ellipsometers can be applied, wherein depending on therespective optical setups, optical paths and measurement principles acorresponding adaptation can occur.

For the measurement the Candela CS20 uses the interaction of a laserbeam having a wavelength of 405 nm which impinges on the wafer surfaceat an angle of 60° relative to the normal. By rotation of the wafer andsimultaneous radial movement of the optical measurement system thecomplete surface of the wafer can be scanned in a spiral pattern. Thesingle intensity measured at each point is depicted by color coding oralso by coding with levels of grey or pseudo-colors in a highly resolvedimage (substrate mapping). FIG. 2 shows typical ellipsometric lateralsubstrate maps, so-called Candela images, for different final cleaningtechnologies. The anisotropic reflection properties of GaAs as well asvariations of the optical properties of the transparent surface oxidelayer, which can for example be caused by contaminations or anon-uniform wet cleaning of the wafers, lead to a locally differingreflection behaviour as a result of changed layer thicknesses and/orindices of refraction. Thereby local fluctuations of the phase shiftoccur which can be quantitatively investigated as a measure for thesurface homogeneity or respectively for the optical homogeneity of theoxide layer.

The radial and azimuthal resolutions can be set in a very large range.For the characterization of large-area GaAs wafers the selection of aradial resolution of 50 μm (or respectively 45 μm at 5 μm beam width)and an azimuthal resolution of 16384 measurement points per trace/track(corresponding to a resolution of 29 μm or respectively 25 μm aftersubtraction of the beam diameter in the outer circumference of a 150 mmwafer or respectively 0.01 μm in the innermost measurement circle) hasbeen found to be appropriate.

The Candela measurement system combines four different detectors for thesimultaneous measurement of the scattering intensity, the topography,the reflectivity and the phase shift of the gallium arsenide substrate.This combination enables a comprehensive characterization and defectdetection with respect to contaminations by process residues, pointdefects, topographical anomalies and surface or respectively (oxide)layer homogeneity of the gallium arsenide substrate. For thecharacterization of the homogeneity of the surface according to thepresent invention the phase shift (“phase shift” channel) is used. Themeasurement principle is explained in more detail, referring again tothe schematic illustration of FIG. 1. For this particular form of anellipsometric measurement the laser beam impinging on the wafer surfaceis polarized in a specific manner. Denoted as Q-polarization, it is amixed form of perpendicularly and parallel impinging beam componentswhich are respectively linearly polarized. Without being bound to thistheory, it is assumed that different reflection behaviour of bothcomponents at the surface of the oxide layer and at the interfacebetween oxide layer and substrate as well as different refractionbehaviour within the oxide layer causes an optical path lengthdifference between s component and p component in the reflected laserbeam leading as a consequence to a phase shift between both components.This phase shift can be determined after the optical splitting of bothcomponents and their spatially separated detection as difference of thesignals of both detectors. Without being bound to this theory, it isfurther assumed that changes of the optical properties of thetransparent surface oxide layer, which can for example be caused bycontaminations or a non-uniform wet cleaning of the wafers, lead to alocally differing light refraction and reflection behaviour as a resultof changed layer thicknesses and/or indices of refraction. Thereby localfluctuations of the phase shift occur which can be quantitativelyinvestigated as a measure for the surface homogeneity or respectivelyfor the optical homogeneity of the oxide layer. In the mapping generatedin the measurement local differences of these layer properties aredepicted as differences in intensity. The high resolution of themeasurement method and the high sensitivity lead to a very exactdepiction of the optical surface properties, which is not or at leastnot usually obtainable other than with the ellipsometers described anddefined here.

In the measurement of GaAs surfaces a crystallographically caused,two-fold anisotropic reflection of the light occurs. The signalvariation associated therewith is superimposed on the actual measurementsignal. In order to improve the sensitivity of the method, the two-foldintensity variations of the background caused by the anisotropicreflection are corrected. For the data tracks recorded for example in aspiral or circular form the two-fold profile is modeled by means ofdiscrete complex Fourier transformation with appropriate frequenciesusing the Levenberg-Marquardt algorithm (see for example J. J. Moré, inG. A. Watson (ed.): Numerical Analysis. Dundee 1977, Lecture Notes Math.630, 1978, p. 105-116) according to the following equation:

  f(?) = ? + a₀cos (4π? + α), ?indicates text missing or illegible when filed

wherein φ is the function variable, α is an angle offset, a₀ is theabsolute offset and a₂ is the amplitude of the model function. FIG. 3presents exemplarily a typical, measured signal curve (raw data) of acircular track (trace), i.e. along a measurement circle as a function ofthe position coordinate, wherein the line “background” describes thecrystal-lographically caused part of the measurement signal which can bemodeled by Fourier transformation using the Levenberg-Marquardtalgorithm. Here, the long-wave, crystallographically caused backgroundoscillation can be separated from the part of the measurement signalstemming from the surface properties by modeling the two-fold profilefor each circularly recorded data track by means of discrete complexFourier transformation with appropriate frequencies using theLevenberg-Marquardt algorithm using the cosine function given above.

FIG. 4 exemplarily shows determined phase shifts of a track aftersubtraction of the crystallographically caused background signal, i.e. atypical curve of the measurement signal along a measurement circle as afunction of the position coordinate after the subtraction of thecrystallographically caused background oscillation is shown. Thiscorresponds to the laterally resolved background-corrected measurementsignal.

The background-corrected mappings now no longer show a two-fold symmetrywhich could interfere with the characterization of deviations of thehomogeneity of the surface properties. This is on the one handillustrated in FIG. 5 for typical ellipsometric lateral substrate maps,so-called Candela images, after subtraction or respectively correctionof the crystallographically caused background signal for different finalcleaning technologies. Inhomogeneities of the surface properties nowemerge more strongly (cf. FIG. 2).

Furthermore, all deviations from the ideal homogeneous surface(residuals of the background-corrected mappings) can be represented in ahistogram. FIG. 6 depicts typical global frequency distributions of thebackground-corrected phase shifts, i.e. residuals of thebackground-corrected mappings, for different final cleaningtechnologies. Since this distribution typically does not correspond to aGaussian normal distribution, the frequently used standard deviationcannot be used as a measure of homogeneity. Rather, it is expedient todefine and to compute appropriate percentiles. For the evaluation of thehomogeneity of the GaAs surfaces according to the present invention thepercentiles P1 or respectively Q_(0.01) (see for example F. Schoonjans,D. De Bacquer, P. Schmid P, “Estimation of population percentiles”.Epidemiology, 22, 2011, p. 750-751) were used for comparisons.

FIG. 7 presents in a comparison typical 1% percentiles of an appropriateplurality of substrates/wafers—here specifically of respectively 25wafers—, which were produced according to the methods of ComparativeExample 1 and 2 as well as Example 1 and 2 described below, wherein thepercentiles are normalized to the respective wafer averages of the phaseshift signal in order to compensate for fluctuations of the irradiatedlight intensity. For the statistical representation so-calledbox-and-whisker plots are used (see for example P. J. Govaerts, T.Somers, F. E. Offeciers, Otolaryngology—Head and Neck Surgery, 118(6),June 1998, p. 892-895 and J. W. Tukey: Exploratory data analysis.Addison-Wesley 1977, ISBN 0-201-07616-0). On the basis of the respective1% percentiles of the single deviations of the phase shift of the lightreflection normalized to the wafer average of the phase shift signals,which can be used for the assessment of the homogeneity of the GaAssurfaces, from FIG. 7 (see also Examples 1-2 and Comparative Examples 1and 2) it can be seen that the so far usual technology produces waferswith surface homogeneities which exhibit 1% percentiles, as definedabove, of less than −0.0065. The differences between the wafersaccording to the invention and the comparative wafers are significantand reproducible. Generally and as demonstrated here in detail, thewafers according to the invention, contrary to comparative wafers, showvalues of greater than −0.0065, preferably greater than −0.0060, morepreferably greater than −0.0055, even more preferably greater than−0.0050, still more preferably greater than −0.0045 and particularlypreferably greater than −0.0040. Even values of greater than −0.0030,preferably greater than −0.0020, more preferably greater than −0.0010and up to 0.0000 excluding 0.0000 come into consideration.

Alternatively the background-corrected mappings can also becharacterized by means of defect classification, for example with theanalysis software made available and correspondingly described accordingto Candela CS20. The mappings show light and dark regions as well asstripe structures which can be assigned to inhomogeneities orrespectively generally “defects”. The intensity and the number of thedeviations from the background characterize the homogeneity of asurface. The intensity differences are classified and counted withrespect to their strength and areal extension. Particular parameters areselected with which the signals to be analyzed and the defects to beanalyzed are defined. In the present case with the analysis software ofthe Candela CS20 in particular the following definitions were made (cf.Table 1 for the present definition of the measurement signals to beanalyzed).

TABLE 1 Neg. de- Pos. de- Kernel Radial Circular viation viation lengthKernel stitching stitching Signal type [%] [%] [μm] type [pixel] [pixel]QAbsPhase 0.05 0.05 1000 median 10 10

The negative and positive intensity oscillations (deviations) are takeninto account from a threshold value of 0.05%. The kernel lengthdescribes the averaging area, the kernel type denotes the kind ofaveraging. In case the positive and negative exceedances of thethreshold values fulfill the distance criteria defined under “radialstitching” and “circular stitching” they are counted as individualdefects and summed-up over the wafer. In this respect the stitchingparameters specify the minimum distance of measurements points whichpositive or negative exceedances must exhibit in order to be counted asseparate defects. Defects whose detected area is greater than 1000 μm²have been found to be relevant. The sum of defects counted in such amanner is a measure for the homogeneity of a wafer. FIG. 8 shows typicalso-called defect maps of background-corrected Candela images fordifferent final cleaning technologies. Such defect maps can serve thedefect classification. For this purpose the analysis software of themeasurement apparatus Candela CS20 is used as described above in whichthe intensity differences of the phase shift are classified and countedwith respect to their strength and areal extension. The sum of suchdefects is a measure for the homogeneity of a substrate or respectivelywafer. For substrates or respectively wafers produced according to sofar usual technology, based on substrate or respectively wafer diametersof 150 mm as reference, defect numbers of >6000 and/or defect areasof >2 cm² are obtained. Substrates or respectively wafers of theinvention which can be produced by the process according to theinvention show, based on substrate or respectively wafer diameters of150 mm as reference, defect numbers of <6000, preferably <5000, morepreferably <4000, even more preferably <3000, still more preferably<2000, still more preferably <1000, still more preferably <500, stillmore preferably <300, still more preferably <250, still more preferably<200, still more preferably <150 and particularly preferably <100,and/or they show defect areas of <2 cm², preferably <1 cm², morepreferably <0.5 cm², even more preferably <0.1 cm², still morepreferably <0.05 cm², still more preferably <0.01 cm², still morepreferably <0.005 cm² and particularly preferably <0.0035 cm².

For a comparison of conventional values and values according to theinvention for defect number and defect area see also FIG. 9 as well asExamples 1 and 2 and Comparative Examples 1 and 2. FIG. 9 shows typicaldefect numbers and defect areas of respectively 25 wafers which wereproduced according to conventional processes (cf. Comparative Examples 1and 2) and according to processes of the invention (cf. Examples 1-2).The differences between the wafers according to the invention and thecomparative wafers are significant and reproducible.

In a further aspect said gallium arsenide substrate can exhibit adiameter of at least 100 mm, preferably at least 150 mm and morepreferably at least 200 mm.

The obtainable advantageous features and properties of the GaAs wafersobtained according to the invention are usable not only for theconventionally used thickness of GaAs wafers, that is approximately inthe range of ca. 600 μm to ca. 800 μm and specifically with theconventional standard thickness of ca. 675 μm (±25 μm). Rather now alsosignificantly thinner and thicker GaAs wafers are accessible.Particularly the altogether more gentle treatment and the veryhomogeneous and significantly defect-reduced surface propertiesaccording to the present invention contribute to this.

In an independent aspect the present invention thus provides for thefirst time finished gallium arsenide substrates with thickness rangeswhich because of the so far not obtainable product properties were nottaken in consideration, namely exhibiting a thickness in the range of ≤approximately 600 μm and alternatively a thickness in the range of ≥approximately 800 μm. More preferable thickness ranges lie for thethinner substrates in a range of approximately 100 up to approximately600 μm, more preferably in a range of approximately 250 to approximately500 μm, or respectively for the thicker substrates in a range ofapproximately 800 to approximately 2000 μm. The diameter of the galliumarsenide substrates according to the invention preferably is at least150 mm. The gallium arsenide substrate products provided according tothe invention are finished (finally processed), i.e. at leastsurface-finished and preferably polished and surface-finished. Thesurface final treatment comprises in particular an oxidation treatmentof at least one surface of the gallium arsenide substrate in drycondition by means of UV radiation and/or ozone gas, a contacting of theat least one surface of the gallium arsenide substrate with at least oneliquid medium and a Marangoni drying of the gallium arsenide substrate.For the polishing of the GaAs surface commonly known procedures can becarried out. Here for the surface final treatment it is referred to thefurther description of the process according to the invention.

The treated surface of the polished and surface-finished galliumarsenide substrate exhibits preferably the surface features alreadydescribed above in connection with the ellipsometric lateral substratemapping with an optical surface analyzer to which reference is madehereby. Finished gallium arsenide substrates processed according to theinvention can therefore alternatively be characterized as follows: (i)by the in comparison to the standard thickness relatively thinner orrelatively thicker layer thickness; (ii) by the properties whichcontrary to the conventional final treatments are obtainable only by thesurface final treatment according to the invention; and (iii) by thedifferences determinable by means of ellipsometric lateral substratemapping with an optical surface analyzer. It is referred to furtherexplanations and definitions given in this application.

The term “approximately” or respectively “ca.” used herein signifiesthat practically a specification of an exact value does not necessarilymatter, rather tolerances of for example ±25 μm are possible, whereinpreferable tolerance ranges are ±20 μm, more preferably ±15 μm, evenmore preferably ±10 μm, and particularly preferably ±5 μm.

The gallium arsenide substrate according to the invention can be dopedor undoped.

Another aspect of the present invention provides a gallium arsenidesubstrate which exhibits at least one surface having within 9 months,preferably 12 months, after the production a substantially notdeteriorating, preferably a not deteriorating, variation of thelaterally resolved background-corrected measurement signal inellipsometric lateral substrate mapping with an optical surfaceanalyzer.

“Substantially” here denotes a change of ≤10%, preferably ≤5%, of thedistribution percentile of 1% normalized to the substrate average of thephase shift signal and/or, based on a substrate diameter of 150 mm asreference, of the defect number and/or the total defect area, whereinreference is made to the preceding explanations and definitions ofdistribution percentile of 1%, defect number and total defect area.

A further aspect of the present invention provides a gallium arsenidesubstrate, wherein the at least one surface within 6 months after theproduction exhibits a substantially not deteriorating, preferably a notdeteriorating, variation of the laterally resolved background-correctedmeasurement signal in ellipsometric lateral substrate mapping with anoptical surface analyzer.

“Substantially” here denotes a change of ≤10%, preferably ≤5%, of thedistribution percentile of 1% normalized to the substrate average of thephase shift signal and/or, based on a substrate diameter of 150 mm asreference, of the defect number and/or the total defect area, whereinreference is made to the preceding explanations and definitions ofdistribution percentile of 1%, defect number and total defect area.

By the process according to the invention not only a very homogeneoussurface is set in a large area, but it is also advantageously maintainedin a very stable manner over a period of time of at least 6 months,preferably 9 months, more preferably 12 months. This is tested andconfirmed by Candela measurements in the course of time afterproduction. In this regard an appropriate storage, in particular astorage of the substrate in darkness under particle-free inert gasatmosphere (e.g. N₂), can contribute to the longer stability of thesubstrate surface.

Another aspect according to the invention is a plurality of galliumarsenide substrates which are produced according to the process of theinvention and which among one another exhibit a substantially same,preferably same, variation from substrate to substrate of the laterallyresolved background-corrected measurement signal in ellipsometriclateral substrate mapping of the respective at least one surface with anoptical surface analyzer.

“Substantially” here denotes a change of ≤10%, preferably ≤5%, of thedistribution percentile of 1% normalized to the substrate average of thephase shift signal and/or, based on a substrate diameter of 150 mm asreference, of the defect number and/or the total defect area, whereinreference is made to the preceding explanations and definitions ofdistribution percentile of 1%, defect number and total defect area.

As a result of the very reproducible process and the possibility ofbatch-wise production a plurality of GaAs substrates is obtained whichamong one another exhibit only a very small variability as regards theirsurface properties. This is tested and confirmed by means ofellipsometric Candela mapping.

A further aspect according to the invention is the use of the galliumarsenide substrate according to the present invention for epitaxialcrystal growth, optionally after storage and preferably withoutpre-treatment after providing the gallium arsenide substrate and beforethe epitaxial crystal growth.

Herein the gallium arsenide substrate can inter alia be used for theproduction of semiconductor devices or electronic and optoelectronicdevices, power components, high-frequency components, light-emittingdiodes and lasers. The excellent surface properties of the substrateaccording to the invention enable the reproducible production of epitaxylayers with high yield.

A further aspect of the present invention relates to the use of anoptical surface analyzer, preferably of an optical surface analyzerCandela CS20, for the optical contact-free quantitative characterizationof the homogeneity of surface properties of gallium arsenide substratesby means of ellipsometric lateral substrate mapping, wherein even morepreferably a laterally resolved measurement signal is corrected from abackground having lower frequency by means of discrete complex Fouriertransformation, preferably using the Levenberg-Marquardt algorithm.

EXAMPLES Material and Methods

Candela ellipsometry: The surface properties of the substrate orrespectively wafer (the properties of the oxide surface on the wafer)are characterized after the final cleaning with an optical surfaceanalyzer (OSA). For the characterization of the homogeneity of thesurface properties the phase shift measurement (“phase shift” channel)of the Candela CS20 from the company KLA-Tencor is used. The measurementprinciple and the measurement configuration have already been describedabove in connection with the ellipsometric lateral substrate mappingwith an optical surface analyzer, to which reference is made herewith(see also FIG. 1). Because of the oblique incidence of the light thelaser irradiates on the wafer an elliptical area with a dimension ofapproximately 5 μm in radial direction and approximately 4 μm in thedirection perpendicular thereto. The fluctuations of the phase shiftbetween the s component and the p component of the reflective laser beamfrom the measurement with the CS20 are used as a measure for the surfacehomogeneity of a GaAs wafer. In the mapping generated by the measurementlocal differences of these layer properties are represented as intensitydifferences.

In the measurement of GaAs surfaces furthermore a crystallographicallycaused, two-fold anisotropic reflection of the light occurs. The signaloscillation associated therewith is superimposed on the actualmeasurement signal. Typical results (“Candela maps”) of ellipsometriclateral mappings of surfaces of differently produced and cleaned GaAssubstrates are shown in FIG. 2. The different values of the phase shiftare presented as light/dark contrast. In the background the typicaltwo-fold light/dark variation stemming from the anisotropy of the lightreflection at the GaAs itself can be discerned, wherein this variationvaries comparatively slowly or respectively exhibits a comparatively low“frequency”. In addition light and dark regions as well as stripestructures can be identified which are in contrast to the backgroundsignal and can be assigned to inhomogeneities or respectively defects.The intensity and the number of the deviations from the backgroundcharacterize the homogeneity of a surface.

1. Candela Measurement

The parameters used for the Candela measurement are presented in Table 2(shows the so-called “recipe parameters” of the measurement, “scanrecipe”) and Table 3 (shows the “wafer setup”). The parameters in thecolumn “scan area” describe the wafer area scanned by the measurementand they are set in Table 3 exemplarily for the dimensions of a 150 mmwafer. The revolution speed of the measurement chuck is set with theparameter “speed”. With the parameter “sampling average” a number ofreplication measurements can be defined from which the measurementresult is determined. With the parameters “step size” the radialmeasurement resolution is set from which results the number of tracks inthe parameter “total tracks”. With the parameter “encoder multiplier”the azimuthal resolution is set. The setting “16×” corresponds to 16384measurement points per track. The radial and azimuthal resolution givenin the lower rows of the column “scan resolution” follows from thesettings for “step size” and “encoder multiplier”.

In the column “laser” the use of the azimuthal laser is predeterminedwhich is required for the measurement of the phase shift as is theQ-polarization of the laser beam in the following column. In the lastcolumn of Table 2, finally in addition particular voltages and offsetsare predetermined, which likewise to the other parameters have proven tobe favorable for the measurements in the context of the presentinvention. When testing different settings for the radial resolution ofthe measurement, it was found that for a track distance between 10 and75 μm there is no influence on the quantified measurement results forthe surface homogeneity. The azimuthal resolution of 29 μm used for themeasurements lies even in the outermost measurement circle over the usedradial resolution of 50 μm.

TABLE 2 Spindle Scan Q- Gain and offsets Scan Area control resolutionLaser Polarization Q-Polarization Start: Speed = 3000 Step size = 50Circumferential Phase Sp1 = 0.5 V; Offset = 47 r = 75000 μm rpm μm Angle= 0° Stop: r = 0 μm, Sampling Total tracks = sp2 = 0.5 V; Offset = 56Angle = 360° average = 1 1501 Encoder PMT circumf. = 400 V multiplier =16x Total Auto PMT offset cir. = 0 resolution: radial = 50.000 μm, TotalPreset Gain Range resolution: angular = 0-28.762 μm

In Table 3 still further parameters for the characterization of the GaAswafers to be measured are summarized which relate to the wafer geometry,the wafer thickness (via the data set saved under “focus”) as well asthe general edge exclusion and a specific edge exclusion for the notch.

TABLE 3 Image Image rotation Wafer Focus angle Analysis area angle roundL_6inGaAs As Start at 68000 μm radius 0 675 μm scanned 150 Notchexclusion: mm l = 1500 μm, w = 3000 μm, Center = 0 und 270° notch

As a result of the Candela measurement a data file is saved whichbesides basic information on the sample and the measurement conditionscontains the measurement data together with the corresponding positioncoordinates in a binary packed format.

2. Background Correction

For the further improvement of the sensitivity of the method it isexpedient to correct for or respectively to adjust for the two-foldintensity oscillations of the background caused by anisotropicreflection. For the spirally or respectively circularly recorded datatracks the two-fold profile is modeled by means of discrete complexFourier transformation with appropriate frequencies using theLevenberg-Marquardt algorithm (see J. J. Moré) according to thefollowing equation:

  f(?) = ? + a₀cos (4π? + α), ?indicates text missing or illegible when filed

wherein ϕ is the function variable, α is an angle offset, a₀ is theabsolute offset and a₂ is the amplitude of the model function (see alsoFIG. 3). The background-corrected measurement signal of a data track isrepresented in FIG. 4. The background-corrected mappings now no moreshow two-fold symmetry (see FIG. 5) which could interfere with thecharacterization of deviations of the homogeneity of the surfaceproperties.

3. Statistical Analysis of the Signal Variations

For the determination of the homogeneity of a substrate surface allsignal variations from the ideal homogeneous surface (residuals of thebackground-corrected mappings) can be represented in a histogram (seeFIG. 6). Since this distribution typically does not correspond to aGaussian normal distribution, the frequently used standard deviationcannot be used as a measure of homogeneity. Rather appropriatepercentiles need to be defined and computed. For the evaluation of thehomogeneity of the GaAs surfaces the percentiles P1 or respectivelyquantiles Q_(0.01) (see for example F. Schoonjans, D. De Bacquer, P.Schmid P, “Estimation of population percentiles”, Epidemiology, 22,2011, p. 750-751) were used for comparisons. Because of possiblefluctuations of the excitation intensity the percentiles are normalizedto the wafer average of the phase shift signal. In FIG. 7 the data arepresented in a comparative manner. Herein box-and-whisker plots areused. The so far usual final cleaning technology produces wafers withsurface homogeneities which exhibit 1% percentiles as defined above ofless than −0.0065. Wafers produced according to the invention here showvalues of greater than −0.0065, preferably greater than −0.0060, morepreferably greater than −0.0055, even more preferably greater than−0.0050, still more preferably greater than −0.0045, and values ofgreater than −0.0040, preferably greater than −0.0030, more preferablygreater than −0.0020, in particular greater than −0.0010 and even up to0.0000 can be taken into consideration.

4. Classification and Counting of Defects

The background-corrected mappings can also be characterized by means ofdefect classification of the analysis software of the Candela CS20. Themappings show light and dark regions as well as stripe structures whichcan be assigned to inhomogeneities or respectively defects. Theintensity and the number of deviations from the background characterizethe homogeneity of a surface. The intensity differences are classifiedand counted with respect to their strength and areal extension.Particular parameters are selected with which the signals to be analyzedand the defects to be analyzed are defined. For the definitions made inthe present case see Table 1 as well as Tables 2-3. The negative andpositive intensity fluctuations (deviations) are taken into account froma threshold value of 0.05%. The kernel length describes the averagingregion, the kernel type denotes the kind of averaging. In case thepositive and negative exceedances of the threshold values fulfill thedistance criteria defined under “radial stitching” and “circularstitching” they are counted as individual defects and summed-up over thewafer. The stitching parameters in this regard specify the minimumdistance of measurement points which positive or negative exceedancesneed to exhibit in order to be counted as separate defects. Thesedefects are only counted if they are greater than 1000 μm². The sum ofsuch defects is a measure for the homogeneity of a wafer (see FIG. 8).For wafers produced according to so far usual technology, based on awafer or respectively substrate diameter of 150 mm as reference, defectnumbers or >6000 or respectively defect areas of >2 cm² result. Wafersproduced according to the invention show defect numbers of <6000,preferably <5000, more preferably <4000, even more preferably <3000,still more preferably <2000, still more preferably <1000, still morepreferably <500, still more preferably <300, still more preferably <250,still more preferably <200, still more preferably <150 and particularlypreferably <100 and/or defect areas of <2 cm², preferably less than 1cm², more preferably less than 0.5 cm², even more preferably less than0.1 cm², still more preferably less than 0.05 cm², still more preferablyless than 0.01 cm², still more preferably less than 0.005 cm², andparticularly preferably less than 0.0035 cm² (see FIG. 9).

5. Roughness Measurement

For the measurement of the roughness in the context of the presentinvention white light interferometry was used. In white lightinterferometry interference images are recorded with a camera, whereinthe interference images result from the superposition of the light fromthe measurement object with the light reflected from a reference mirror.For a topography measurement the z position of the objective is adjustedin small steps and at each position an interference image is recorded.An image stack is obtained from which the height data are computed. Byusing a white light source with short coherence length, surfaces can becaptured with very good height resolution as is known forinterferometric measurement methods. For the measurements of theroughness in the context of this invention the apparatus NewView 5022Sfrom the company Zygo was used. The measurements were performed with anapparatus with an objective with 20× magnification, The measuring fieldsize was 180×130 μm. The given roughness R_(a) is the difference betweenthe maximum and minimum height value for the given measurement fieldsize.

6. Determination of the Etching Abrasion

For the determination of the etching abrasion in between the dryoxidation and the wet cleaning according to the different embodiments ofthe present invention a chemically resistant adhesive tape was adheredto the front side of a GaAs wafer. After the cleaning process thisparticular adhesive tape was detached in a residue-free manner and theheight of the formed step was measured by means of the white lightinterferometer according to the above-mentioned method at 5 points,

Comparative Example 1

A GaAs wafer after the last polishing step is subjected to a basiccleaning with a 0.5% NH₄OH solution and an acidic cleaning with a 5% HFsolution for the removal of metallic contaminations. Subsequently theparticle removal from the wafer surface is carried out by a brushscrubbing process. The cleaning procedure of the GaAs wafer is finishedby a rinsing with deionized water and the drying by means of spindrying. After this conventional process for the surface cleaning of theGaAs wafer in the measurement of the surface homogeneity with themeasurement apparatus Candela CS20 using the above-described recipes forthe measurement and defect analysis, a variation of the laterallyresolved background-corrected measurement signal is found whosedistribution percentile of 1% normalized to the wafer average of thephase shift signal is less than −0.0065, as well as, based on a 150 mmGaAs wafer as reference size, a defect number of greater than 6000 and atotal defect area of greater than 2 cm² on the surface is found (seeFIGS. 6-9).

Comparative Example 2

Subsequent to a conventional cleaning and drying as described in thefirst Comparative Example, a GaAs wafer is subjected to an oxidationprocess. The oxidation is carried out by irradiating the whole area ofthe wafer surface by means of short wavelength UV light (wavelength220-480 nm, power 20-40 mW/cm²), while slowly rotating the wafer, fore.g. one minute. Subsequently the wafer is subjected in a process rackto a basic cleaning in a 0.5% NH₃ solution under the influence ofmegasound, it is subsequently rinsed in the overflow and then removedfrom the process rack and dried by means of spin drying at 2500 rpm. Themeasurement of the surface defects on thus cleaned substrates with themeasurement apparatus Candela CS20 leads to a variation of the laterallyresolved background-corrected measurement signal whose distributionpercentile of 1% normalized to the wafer average of the phase shiftsignal is less than −0.0065, to more than 6000 individual defects and atotal defect area of greater than 2 cm² on the surface of the 150 mmwafer as reference (see FIGS. 6-9).

Example 1

After the conventional cleaning procedure as in Comparative Example 1, aGaAs wafer is subjected to the further steps of dry oxidation, NH₄0Hcleaning and DI water rinsing as in the Comparative Example 2. Incontrast to the Comparative Example 2, however the wafer is not dried bymeans of spin drying, but according to the Marangoni process. Themeasurement of the surface defects with the measurement apparatusCandela CS20 gives a variation of the laterally resolvedbackground-corrected measurement signal whose distribution percentile of1% normalized to the wafer average of the phase shift signal is greaterthan −0.0065. Furthermore, less than 100 individual defects and a totaldefect area of less than 2 cm² have resulted on the GaAs surface of the150 mm wafer, and thereby the superiority of this cleaning processcompared to the conventional cleaning in Comparative Example 1 and evencompared to an improved cleaning but with conventional drying inComparative Example 2 is shown (see FIGS. 6-9).

Example 2

The cleaning of a GaAs wafer is initially carried out in a conventionalmanner as in the Comparative Example 2. Subsequently a dry oxidation anda basic cleaning with subsequent DI water rinsing are carried out as inthe Comparative Example 2 or respectively in Example 1. Subsequent tothe rinsing with DI water in this embodiment of the present invention afurther acidic cleaning step in combination with ozone dissolved in theliquid is carried out. In the rinsing with DI water subsequent to theacidic cleaning step furthermore the addition of an acid or base iscarried out in order to advantageously prevent the formation of aninhomogeneous oxide layer during the rinsing. Using 0.2% HCl and 50 ppmozone in the acidic cleaning step over a process time of 3 minutes andusing the subsequent DI water rinsing with addition of a small amount of25% NH₃ solution, the measurement of the surface homogeneity with themeasurement apparatus Candela CS20 carried out subsequently to theMarangoni drying leads to a variation of the laterally resolvedbackground-corrected measurement signal whose distribution percentile of1% normalized to the wafer average of the phase shift signal is greaterthan −0.0065. Furthermore, less than 100 defects and a total defect areaof less than 2 cm² on the surface of the 150 mm GaAs wafer are found(see FIGS. 6-9).

In case the last DI water rinsing is performed without the addition of apH value modifying substance, the defect level measured after thecleaning process can increase to above 2000 defects on a 150 mm GaAswafer.

Example 3

When in the treatment according to Example 2 instead of ozone hydrogenperoxide is used as oxidizing agent in the acidic cleaning step, theCandela measurement after the Marangoni drying gives above 4000 defects.

1. A gallium arsenide substrate which exhibits at least one surfacehaving a surface oxide layer comprising gallium and arsenic oxides andwhich exhibits at least one surface having, according to anellipsometric lateral substrate mapping with an optical surfaceanalyzer, based on a substrate diameter of 150 mm as reference, a defectnumber of <6000 and/or a total defect area of less than 2 cm², wherein adefect is defined as a continuous area of greater than 1000 μm² having adeviation from the average measurement signal in elipsometric lateralsubstrate mapping with an optical surface analyzer of at least ±0.05%.2. The gallium arsenide substrate according to claim 1, and having adiameter of at least 100 mm.
 3. The gallium arsenide substrate accordingto claim 1, wherein the diameter is at least 150 mm.
 4. The galliumarsenide substrate according to claim 1, wherein the diameter is atleast 200 mm.
 5. The gallium arsenide substrate according to claim 1,wherein the at least one surface within 6 months after the productionexhibits a substantially not deteriorating variation of the laterallyresolved background-corrected measurement signal in ellipsometriclateral substrate mapping with an optical surface analyzer.